This invention relates to an electrochemical cell of the concentration type which is thermally regenerative using a low grade heat source. Thermally regenerative electrochemical cells have previously been suggested as topping devices for nuclear reactors to convert heat from reactors operating at temperatures higher than can be tolerated in steam cycles. One such cell was based on the lithium hydride system. Lithium hydride thermally decomposes to hydrogen and lithium at temperatures in the range of about 900.degree. C. to 1200.degree. C. and thereafter is recombined in the fuel cell. The fuel cell has a molten salt electrolyte which dissolves the lithium hydride and serves as a transparent medium, the cell operating at a temperature of about 500.degree. C. and having potential of between about 0.3 and 0.4 volts. The design electrical efficiency of the lithium hydride system is about 15% and 17%.
Another example of a thermally regenerative electrochemical cell is the sodium amalgam system in which mercury is distilled off the amalgam and returned to the dilute side of the concentration cell. The concentration cell and the distillation column or still are operated at about 500.degree. C., the cell having a potential of about 0.4 volts and the design efficiency being about 7%. In one example the electromotive force of the electrochemical cell is derived from the free energy of formation of a compound, lithium hydride, and in the other example the electromotive force is derived from the concentration differences in the sodium amalgam system.
It is desirable to provide a thermally regenerative cell operative with low grade heat sources, such as solar energy from a solar collector or low grade waste heat from industrial processes. Optimally, the cell should be designed to operate at temperatures below 400.degree. C. and preferably less than 300.degree. C. Both the lithium hydride system and the sodium amalgam system previously discussed operate at temperatures substantially above the optimal temperatures for low grade heat sources.
Representative literature in the field includes U.S. Pat. No. 3,671,322 issued to L. A. King et al., June 20, 1972 for Electrochemical Cell with Aluminum Electrodes and Different Electrolyte Concentrations in Two Compartments, the King et al. patent describing a concentration cell having aluminum electrodes, an aluminum chloride-sodium chloride electrolyte activating at temperatures in the range of about 125 to about 180.degree. C., the cell voltage being about 0.2 volts.
The Doundoulakis et al. U.S. Pat. No. 3,414,437 issued Dec. 3, 1968 for Fluid Circulating Battery System describes cells using separate anodic and cathodic mixtures which are collected for disposal after use. U.S. Pat. No. 3,887,399 issued June 3, 1975 to Seiger for Method for Producing Electrical Energy with Consumable Aluminum Anode describes an activated aluminum anode alloyed with mercury with an aqueous electrolyte in which the cathode is of porous carbon or a porous metal. The activated aluminum anode may be activated by the addition thereto of metals such as gallium, cadmium, indium or thallium.
U.S. Pat. No. 4,064,327 issued Dec. 20, 1977 to King et al. for Aluminum-Chlorine Thermal Battery describes a concentration cell with an aluminum anode, an inert metal cathode and a two-layered pelletized electrolyte sandwiched therebetween, the electrolyte being a combination of aluminum chloride and one or more of sodium chloride, lithium chloride, potassium chloride and the tetra substituted ammonia chloride salts. Cell potentials of about 0.6 volts are reported. U.S. Pat. No. 4,145,483 issued Mar. 20, 1979, to Bonnemay et al. for Accumulators Including Halogen Electrodes Operating With Fused Halide Based Electrolytes discloses accumulators of aluminum/aluminum halide mixtures and alkaline metal halide and metallic derivatives in a graphite matrix.